The present invention relates to a method for producing diketones of a type which are themselves useful in the production of certain tetraketones which may be used for the manufacture of thermally stable quinoxaline polymers.
Polyquinoxaline resins have become widely sought after in recent years due to their unique thermal properties. Because of their desirable properties, the polymers are useful for high temperatures adhesives, coatings and films in a number of critical industries. As stated in Stille, U.S. Pat. No., 3,661,850, polyquinoxaline polymers are "suitable for high temperature electrical insulators, battery separators, foams, ablative materials for re-entry bodies and rocket nozzles." Other patents which discuss these unique resins, their properties and uses are: Augl, U.S. Pat. Nos. 3,766,141 and 3,642,700; and Hergenrother, U.S. Pat. No. 3,778,412.
Stille, Augl and Hergenrother all disclose methods of preparing polyquinoxalines from tetraketones of the following general formula: ##STR3## In Stille R is said to be selected from the group consisting of alkyl, aryl, alkaryl, and aralkyl groups and hydrogen, while R' is selected from the group consisting of alkylene, arylene, aralkylene and alkarylene groups (Col. 3, lines 6-16). Similarly in Augl, R is listed as C.sub.6 H.sub.5 or H and R' is m-phenylene, p-phenylene or a number of diphenyl compounds (Col. 2, lines 29-64 of the U.S. Pat. No. 3,766,141). In Hergenrother, R is stated to be hydrogen, alkyl, phenyl and substituted phenyl and R' is for the most part selected from a number of divalent alkyl, phenyl and diphenyl compounds (Col. 3, lines 1-34).
These prior art patents list several methods of preparing the desired tetraketone precursors used in manufacture of the polyquinoxaline resins. Included are methods which utilize a diketone having the formula ##STR4## wherein R and R' are as defined above by the patentees in regard to the tetraketones. Thus, Hergenrother and Augl both disclose methods wherein diketones are oxidized to tetraketones with selenium dioxide or selenious acid. (Hergenrother, Examples 3-5; Augl, Col. 3, lines 3-36).
The diketones themselves are said to be produced by any number of methods. For example, Augl in U.S. Pat. No. 3,766,141 discloses a reaction in which certain diketones are prepared by reacting dicarboxylic acid derivatives under standard conditions with thionyl chloride to form the acid chloride compound. The acid chloride compound is then reacted with benzene and aluminum chloride under typical Friedel-Crafts conditions to obtain a phenylacetyl compound (Col. 3, lines 3-36).
See also, Hergenrother (U.S. Pat. No. 3,778,412) which suggests preparation of p,p'-diacetyldiphenyl ether by Friedel-Crafts acetylation of diphenyl ether in methylene chloride (Example 3); preparation of p,p'-di(phenylacetylphenyl) ether by mixing diphenyl ether and phenylacetyl chloride in methylene chloride, adding that mixture to a suspension of anhydrous aluminum chloride in methylene chloride under nitrogen, and separating and recrystallizing (Example 4); and preparing p-di (phenacyl) benzene from a benzene solution of the diacid chloride of p-phenylenediacetic acid which was added to a slurry of anhydrous aluminum chloride (Example 5).
The use of Friedel-Crafts reactions for production diketones is, of course, commercially feasible. In fact, the disclosed systems are quite popular. However, in a number of instances the costs are excessive because of the expense in obtaining the starting materials. Likewise, these reactions do not offer a large degree of flexibility in the steps involved or the production of some of the moe exotic diketones.
It is also known that certain other diketones of this type may be obtained via the reactions of the appropriate dinitriles with Grignard reagents. However, this process is expensive due to the fact that, although the reactions are run in very dilute solutions, a large percentage of the Grignard reagent is lost through coupling. In fact, only a few diketones of this type have been synthesized because several functional groups, such as amino groups, hydroxyl groups, nitro groups, etc., react with Grignard reagents and, therefore, cannot be present in the starting materials.
Accordingly, the practical usefulness of the prior art process has been considerably limited. Therefore, the need exists for an improved process for producing large quantities of diketones as inexpensively as possible.